Diagnosing a slow fuel leak

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Me and my housemate were working on my 74 Dart this evening, and he recommended I check for a fuel leak, based on the heavy smell of gasoline while the car was running.

The problem is I've never found puddles or other signs of a leak anywhere. I would not be surprised if there was a slow leak somewhere in the system.

Might you guys have any advice on trying to diagnose a leak?
 
A very small leak can vaporize and disappear rather than drip. A leak at the fuel pump drips on the K member. Hard to see behind/below the alternater too.
If I couldn't see the leak at the carb area I would disconnect the battery and set the alternater out to look at the fuel pump area.
 
You could also just be running really rich. I have the gasoline smell in my garage after driving mine. Mine is from gas vaporization (boiling) in the fuel filter and carb.
 
Check the entire gas line for "clean" spots where leaking gas might wash dust and grime away. The most common places for leaks are near the clamps.

If you've ruled out that you might be running rich like mentioned above, and still cannot find a leak, you might want to replace the entire gas line. It shouldn't cost more than $40.
 
It might be the carb bowl vent that goes to the evaporative canister. Check for leak or disconnect on that hose, or other hoses at the canister. There is a vent form the tank that goes there too.
 
First thing I would ask is have you noticed the fuel level running down after sitting for a while? Also check the filler neck grommet, it could be old/shrunk/cracked/missing allowing gas to evaporate from the tank or fuel to slosh out while in motion (which you wouldn't see when examining the parked vehicle).

Do this - back down the driveway a few feet and hit the brakes hard so it's a hard stop. Put it in park and real quick, while the gas in the tank is still sloshing around, run back and see if you see wetness anywhere around the tank.
 
My car was giving me a bad smell too and I was convinced it was coming from under the hood. I found out later it was the donut gasket at the filler neck leaking (it had split). It never did drip, it just smelled bad. Anyhow, I dropped the tank, replaced the gasket and have never had a gas smell since.
 
Check the two rubber fuel hoses that connect the gas tank to the fuel lines. Odds are they've never been touched & may be cracked at the ends. Replace them with fuel injection style hoses for more longevity.
 
Have you checked the fuel line where it connects to the carb? Also, if you have the plastic clear fuel filter (replace it with a metal one) but check if it holds fuel after the car has been sitting.
 
roadrunnerh said:
Have you checked the fuel line where it connects to the carb? Also, if you have the plastic clear fuel filter (replace it with a metal one) but check if it holds fuel after the car has been sitting.
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First off, thanks guys for the responses. If you haven't my posts on my car before, she gets really bad gas mileage (avg ~8 mpg using premium fuel), and I've slowly been trying to address everything that could cause this, especially since gas is about to hit $4/gal in PHX.

The carburetor is a new Edelbrock. I replaced the fuel filter a few weeks ago to one of the plastic ones, and I want to put a metal one back on (hooray for getting a paycheck again). I just replaced the distributor and did the timing last night (currently at 10* btdc).

My next step will be doing the fuel line, as my housemate will be redoing his 76 F150 in the near future as well. Time just isn't conducive for that with work right now. I'll check out some of the things you have brought up and post pictures, maybe you'll be able to give me some more pointers then.

Thank you for the help, I'm definitely not the most mechanically inclined guy around.

Rick
 
I find it hard to believe that a fuel line could have rusted thru in AZ. That would be rare even in the rusty north. Check the rubber hoses at the tank and fuel pump, and replace if you see any cracks on the outside. Use "fuel injection" hose. It costs more but doesn't degrade in ethanol. My guess is that your 74 has a charcoal canister and such. Those are notorious for failing and giving gas smells. Check all the hoses to and from it.

You should get much better than 8 mpg. I assume this is a small block (318 cu in). You should be able to get at least 15 mpg hwy since you have electronic ignition. But not if it has a wild racing cam (blub-blub idle). That will give all kinds of nasty smells at idle, noise, and waste gas, but many people like that. I get 18 mpg hwy in my 4000 lb C-body w/ a big block 383 engine.
 
BillGrissom said:
I find it hard to believe that a fuel line could have rusted thru in AZ. That would be rare even in the rusty north. Check the rubber hoses at the tank and fuel pump, and replace if you see any cracks on the outside. Use "fuel injection" hose. It costs more but doesn't degrade in ethanol. My guess is that your 74 has a charcoal canister and such. Those are notorious for failing and giving gas smells. Check all the hoses to and from it.

You should get much better than 8 mpg. I assume this is a small block (318 cu in). You should be able to get at least 15 mpg hwy since you have electronic ignition. But not if it has a wild racing cam (blub-blub idle). That will give all kinds of nasty smells at idle, noise, and waste gas, but many people like that. I get 18 mpg hwy in my 4000 lb C-body w/ a big block 383 engine.
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Car is originally from California, for what it is worth. Yes the engine is a 318, the guy who had the car before me said it was his daily driver, so I don't believe it has been given any racing parts.

Up next, hopefully in two weeks, is the timing chain. I know I've read on here that timing chains wearing out was a common problem with the 318.

Thanks for the advice.
 
Bad valve timing is a good guess. You can fairly easily check for excessive slop in the timing chain by watching for dead-band in the distributor rotor movement when you manually turn the engine over in the forward then reverse direction. Should be <10 deg on the crank damper. The 318 from the factory might have had a nylon gear on the camshaft. My 65 big block did. The chain can skip a tooth on those after 40 years.
 
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